Materials based on hydroxyl-terminated polydimethylsiloxane cross-linked via a boron-containing compound have found broad use and can be found in various patents and patent applications. Applications for these materials range from the automotive industry, for example, as resonance dampers, through therapeutic and rehabilitative uses, to uses as children's toys. They are also used in households and by hobbyists to remove dirt from various surfaces and as a masking medium when spray-painting.
U.S. Pat. No. 2,541,851 represents early work in the field and describes the use of various compounds of boron including pyroboric acid, boric anhydride, boric acid, borax and hydrolyzed esters of boric acid to form a solid, elastic product with a dimethyl silicon compound having two hydrolysable groups that have been hydrolyzed. Another material is described in U.S. Pat. No. 5,607,993 which discloses a so-called bounce clay. The bounce clay contains a proportion of particles having a filler function with the aim of lowering the density of the clay product. Another known material is described in WO 2008/020800 A1 which discloses a particulate or granular material with a silicone-based binder which is disposed as a coating on the particles or the grains.
However, it has recently been concluded that boron-containing compounds may have an adverse effect on the health of humans. Such compounds are therefore now classified as toxic for reproduction in the European Union, and the following boron-containing compounds have been added to the European Chemicals Agency (ECHA) candidate list: Diboron trioxide (CAS: 1303-86-2); Tetraboron disodium heptaoxide, hydrate (CAS: 12267-73-1); Boric acid (CAS: 10043-35-3, 11113-50-1); Disodium tetraborate, anhydrous (CAS: 1303-96-4, 1330-43-4, 12179-04-3).
If a substance on this list is present in a product in a concentration above 0.1 wt. % then that product will be subject to certain restrictions and the supplier will have obligations in relation to its safe use. Since some of the product substances can be difficult to analyze via common analytical techniques it is useful to recalculate the limitation in terms of the corresponding amounts of boron. An elemental analysis of boron content is usually straightforward to perform.
0.1 wt. % of diboron trioxide (CAS: 1303-86-2); Tetraboron disodium heptaoxide, hydrate (CAS: 12267-73-1); Boric acid (CAS: 10043-35-3, 11113-50-1); Disodium tetraborate, anhydrous (CAS: 1303-96-4, 1330-43-4, 12179-04-3) correspond to 0.031 wt. %; 0.020 wt. %; 0.017 wt. % t; 0.021 wt. % of boron, respectively.
Furthermore, following the European toy safety directive (2009/48/EC), boron is not allowed above 0.03 wt. % in certain product classes and not above 0.12 wt. % in certain other product classes. In all, recalculated to boron, the strictest limitation (boric acid) corresponds to 0.017 wt. % boron.
These limitations greatly hamper the use of materials based on hydroxy-terminated polydimethylsiloxane cross-linked via boron-containing compounds in many applications, and exclude their use in other applications, since the levels of boron are too high. For example, U.S. Pat. No. 2,541,851 claims a working range of the boron compound of between 5-25 wt. % based on the weight of the polymeric dimethylsiloxane, and U.S. Pat. No. 3,177,176 claims 1-10 wt. %.
Thus it is of great concern to find a silicon-containing composition having a boron content which can meet the regulations, i.e. is below the stipulated amounts, whilst maintaining the required properties of the compounds currently used in the art.
Various ways to manufacture the composite materials of the art, which are all characterized by having a boron content outside the scope of the new regulations, are known to the skilled worker. Often the process involves “cooking” the mixture for several hours. During the cooking operation a nitrogen blanket is spread over the mixture to prevent volatiles from burning or exploding. However, retaining the volatiles may result in a product which cannot demonstrate the necessary physical properties, for example having inadequate strength and/or rebound capability.
U.S. Pat. No. 4,371,493 describes a process based on dimethyl silicone gum which is claimed to result in lower rejection frequency. However, this process also requires heating to 150-260° C. for several hours, and the addition of boron compound in the range of 4-15 wt. %, which is above the amounts allowed by new European regulations.
U.S. Pat. No. 3,177,176 describes that it is preferable first to mix all components in a state of low viscosity, followed by an increase in temperature to between 90-250° C. until there is a sudden and substantial increase in viscosity. The working range of the boron compound is between 1-10 wt. %, which is again clearly above the amounts allowed by new European regulations.
US2012/0329896 discloses a process which comprises the addition of low levels of a boron-containing crosslinking agent to a polyorganosiloxane to form a borosilicone compound, crosslinking the resultant composition with a siloxane crosslinking agent, and curing the resultant mixture to form a viscoelastic silicone rubber composition.
However, compositions therein described have a permanent equilibrium shape, meaning that they will return to an equilibrium shape after being deformed. In addition, the curing time for such compositions is a period of several days.
Another general disadvantage with previously known manufacturing processes is the excess heating and prolonged reaction time that is needed before the reaction is completed. It would be a substantial advantage if the increase in viscosity could be initiated in a controlled and convenient way. It would also be desirable for the increase in viscosity to commence shortly after the initiation of the process, and furthermore without excessive heating.